Grid control gaseous discharge rectifier tube



Nov. 22, 1955 w. P. KRUGER ,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 4Sheets-Sheet l 11 1O F|G|.1. I 75 44 "60 EM 435 46 i i R i INVENTOR.

BY WP Kruger WMMU,

H is ATTORNEY Nov. 22, 1955 w. P. KRUGER 2,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 4Sheets-Sheet 2 FIG.2.

52 ll EQQ 2626 25 I 19 E:- =f s l 24 INVENTOR. BY W P. Kruger HisATTORNEY Nov. 22, 1955 w. P. KRUGER 2,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 v 4Sheets-Sheet 5 IN V EN TOR.

BY W. P Kruger MM m,

His ATTORNEY Nov. 22, 1955 w. P. KRUGER 2,724,786

GRID CONTROL GASEOUS DISCHARGE RECTIFIER TUBE Filed Jan. 26, 1952 4Sheets-Sheet 4 IN VEN TOR.

BY WP Kruger WM m,

H is ATTORNEY United States Patent GRID CONTROL GASEOUS DISCHARGERECTIFIER TUBE Application January 26, 1952, Serial No. 268,390

20 Claims. (Cl. 313-29) This invention relates to controllable gaseousdischarge tubes, and more particularly to a tube structure suitable forgrid control gas tubes of the air-cooled mercury pool type.

In certain types of gaseous discharge rectifiers, the tube is normallyin condition to conduct when its anode is made positive, and a controlelement commonly called a grid serves to hold off or permit conductionthrough the tube as and when desired. A tube having a mercury poolcathode may be maintained in such normal condition for conduction by akeep-alive anode and suitable starting device, as distinctive from thetype of mercury pool tube, commonly known as an ignitronQin whichconduction is initiated by an ignitor.

In tubes of this type, in order that the grid may effectively exerciseits controlling influence for high positive anode voltages, there shouldbe no paths for efiective ionization at such anode voltages which arenot subject to the controlling influence of the grid, otherwise in spiteof the control of the grid, the tube may conduct at the higher anodepositive voltages, and impose an undesired limitation for its peakforward voltage rating. In this connection, the gaseous ionizable mediumis present in all spaces and regions inside the tube envelope, andionizing paths to initiate an unwanted discharge current are not limitedto the surface of the anode opposite the control grid, but may extend tothe back surface of the anode, its supporting lead, or any part at anodepotential.

With these considerations in mind, one object of the invention is toprovide a shielding structure, effectively insulated from the anode andits supporting elements, which completely shields the anode andassociated parts at anode potential in such a manner that all paths forelfective ionization and initiation of conduction current to the anodeare subject to the controlling influence of the grid, thereby enabling amuch higher peak forward voltage rating to be obtained.

Generally speaking, and without attempting to define the nature andscope of this invention, it is proposed to provide such shieldingfunctions by a metal shield at cathode or other appropriate potential,which is electrically isolated by a special structural arrangement ofparts from the anode and its supporting lead-in connection, and which isdisposed in such closely spaced relation to all surfaces of anodepotential, except for a region opposite the control grid, that there isno path for electron movement as between the shield and a point of anodepotential sufliciently long for effective ionization and initiation of adischarge current to the anode not under the control of the grid. Morespecifically, the invention involves a structure for the joint orconnection between such shield and the anode lead adjacent its seal inthe tube envelope, which will provide adequate electrical isolation forthese parts for high anode voltages, and yet avoid any long paths forionization between the shield and anode lead.

Other objects of the invention are to provide improved structures forthe control grid, keep-alive anode, starting device and other elementsto be used in connection with 2 a completely shielded anode in a tube ofthe mercury pool type, using an envelope structure of metal and glass ina manner to obtain a grid control mercury pool rectifier tube ofacceptable shape and size and peak forward and inverse voltage ratings.

Various other objects, characteristic features, attributes andadvantages of the invention will be in part apparent, and in partpointed out, as the description advances.

Although the invention may be embodied in various specific structuralforms, it is convenient in describing the nature of the invention andits characteristic features to refer to a tangible physical embodimentof the invention, such as the typical tube structure illustrated in theaccompanying drawings.

In these drawings,

Fig.1 is a side view, partly in section, illustrating the generalstructural organization of one physical embodiment of the invention.

Figs. 2, 3, 4 and 5 are transverse sections through the tube taken onthe lines 2-2, 3-3, 4-4 and 55 indicated in Fig. 1.

Fig. 6 is a fragmentary view on the line 6-6 in Figs. 1 and 3,illustrating a part of the structure for providing an external lead-inconnection to the control grid; and

Figs. 7 and 8 are diagrammatic views illustrating the circuitconnections associated with the keep-alive anode and automatic startingdevice.

Considering the structural features of the specific embodiment of theinvention illustrated as applied to a mercury pool type of gaseousdischarge rectifier tube, the envelope comprises a glass body E ofgeneral cylindrical form, with electrode supports and lead-inconnections sealed in its upper end, and a circular metal bottom or baseB, which is sealed to the lower edge of the glass body E, supports thepool of mercury constituting the cathode and indicated at 2, and hasattached thereto an external radiator R, in accordancev with theenvelope structure disclosed and claimed in the application of AndrewHumphrey, Ser. No. 268,368, filed January 26, 1952. The purposes andadvantages of an envelope of glass and metal in this form for mercurypool tubes are explained in this Humphrey application; and no claim ismade herein to such an envelope and radiator structure by itself.Generally speaking, and for the purposes of explanation in this case, itmay be said that the metal bottom B and radiator R serves to maintain aregion at and around the mercury pool cathode inside the envelope at theappropriate temperature'by air cooling to establish the vapor pressurewithin the envelope for the desired voltage rating for the tube, all inan envelope of convenient size and with small over-all dimensions forthe current rating of the tube.

In the tube envelope structure of this type, for effective heatdissipation the bottom or base B should have substantial area, berelatively thin, and be formed of a metal which is not affected bymercury and which is capable of forming a gas tight seal with the glassbody E of the envelope. For these reasons'the circular base B is a thinsheet of metal or alloy, which is capable of forming a gas tight sealwith the glass, such as the alloy commonly known as Kovar. I have foundthat when such a Kovar disc is sealed in the ordinary way at itsperiphery to the glass body of the envelope, deformation of this thinmetal disc after seal has been made by atmospheric pressure when theenvelope is exhausted, is likely to produce damaging stresses upon theglass-to-metal seal. Accordingly, the base B of the envelope structureof this invention is formed with a peripheral trough 3 pro+ portioned toleave a space between the inner wall of the trough and the inner edge ofthe glass body E of the envelope when the glass is sealed to the bottomof this trough. In such a structure, the radial stresses at theperiphery of the base B as it is deformed by atmospheric pressure oruneven expansion will be absorbed, so to speak, by bending of the metaladjacent the inner wall of the trough, and will not be applied to theseal itself. This structural arrangement permits the sealing of a metalbase of the desired area and thinness to the glass body E of the tubeenvelope in a simple and effective manner, without having excessivestresses applied to the seal itself by deformation of this metal baseafter the seal has been made.

The tube elements enclosed and supported in this envelope structure,together with the appropriate lead-in connections, comprise in generalan anode A and its supporting lead-in rod 5, control grid G, a metalshield S around the anode and grid with an arrangement of baflie plates,keep-alive anode KA, and an automatic starting device in the form of aflipper armature to be actuated by an electromagnet EM suitablysupported outside of the envelope.

Considering the structural features of these tube elements, the anode Ain the particular arrangement shown comprises a disc of carbon formedwith a flat bottom surface, a beveled top surface, and a screw-threadedhub 6 to receive the threaded end of an anode supporting and lead-in rod5, preferably provided with a locking nut 7. The upper end of the anodesupporting rod is suitably fastened, such as by resistance welding, tothe inner surface of a flanged cup 10 of Kovar or like alloy, capable offorming at its periphery a gas-tight seal with the glass of the bodyE ofthe tube envelope. The external circuit connection to the anode A isconveniently made by a flexible cable or the like 11 attached to theoutside of the cup 10. A thin metal tube 12 fits closely at its lowerend around the hub 6 of the anode, and the upper end of this tube fitsinside the flange of the cup 10, for purposes later explained.

In the tube structure of this invention, for reasons more convenientlydiscussed later, all surfaces of the anode A and the metal tube 12around its supporting lead 5, except for an area at the bottom face ofthe anode opposite the control grid G, are completely shielded by aclosely spaced imperforate sheet metal shield, designated as a whole S.The upper part of this metal shield S, conveniently termed an anodeshield, is shaped to be closely spaced, say in the order of 2 mm, to theback or top surface of the anode A, to the tube 12 around its supportingrod 5, and to the outer periphery of the grid G.

A plurality of heat radiating fins 14 (seven as shown) of convenientshape, as illustrated in Figs. 1 and 2, are preferably secured by weldedflanges to the top of the shield S to facilitate heat dissipation. Theanode shield S as a whole is rigidly supported by a plurality ofrelatively stiif rods 15, at least three in number, to form athree-point support. These rods 15 are welded at their lower ends tocertain of the heat radiating fins 14, and are anchored at their upperends in conventional seals (not shown) in the end of the glass body E.One of these supporting rods 15 is preferably extended through itsanchoring seal through the glass body E to constitute a lead-inconnection to permit the shield S to be readily connected to thecathode, or carried at some other potential base adapted for theoperating conditions. In this connection, it is not necessary in somecases to have such an external connection to establish a fixed potentialfor the shield S, which may be a floating electrode assuming thepotential of the plasma in operation.

The control grid G comprises, as shown in Figs. 1 and 3 a plurality offiat grid bars 18 of suitable heat resistant metal, which are disposededgewise to the anode A, and are supported on a flanged grid ring 19,One important feature of the grid structure is that each grid bar 18 isrigidly attached at one end to the flange of the grid ring, as by Weldedtabs 20 (see Fig. 3), while the other end of each grid bar fits betweenbut is not attached to abutments or pillars in the form of L-shapedpieces 21 Welded to the flange of the grid ring. These abutments 21retain the ends of the grid bars 18 in the desired space relation, butpermit a sliding motion of these ends to permit the grid bars to expandor contract individually upon changes in temperature after the grid isassembled, without the tendency to bend the grid bars transversely anddistort their space relationship that would otherwise occur if the gridbars were rigidly attached at both ends to the grid ring 19.

The grid G is supported and insulated from the inside wall of the anodeshield S by a plurality of insulating supporting elements, three asshown, with the outer edge of the grid ring closely spaced to the wallof the shield S. In the construction shown, which involves the featuresof the insulator structure disclosed in the prior patent to E. K. Smith,No. 2,456,540, December 14, 1948, a bent rod 23 is welded at its upperend to the inside of the grid ring 19, and extends through a pair oftubular insulators 24, 25 of steatite or like heat resistant insulatingmaterial and having recessed ends in accordance with the disclosure ofsaid Smith patent. These tubular insulators 24, 25, one disposedvertically and the other horizontally, are secured to the inside wall ofthe shield S by metal bands 26 welded thereto and seated incircumferential grooves in said insulators. After the parts areassembled, the grid G is effectively supported in the desired closelyspaced relationship to the anode A and the inner wall of the heat shieldS. The horizontally disposed insulator 25 limits up and down movement ofthe grid, and the vertically disposed insulator 24' limits sidewise andturning movement of the grid.

The external lead-in connection to the grid G comprises a relativelystiff wire 28 passing through a conventional seal (not shown) in theupper end of the glass body E of the tube envelope, and this wire 28extends downward to a point where it passes through a hole in the anodeshield S and is attached by a welded washer to the grid ring 19, asindicated at 29 in Fig. 6. In order to avoid ionic current to this gridlead, it is electrically insulated throughout its length by tubes 36 cfsteatite, with the joints between the tubes covered with a ceramiccement. The tubular insulator 31 around the wire 23 where it passesthrough the shield S, as best shown in the fragmentary view in Fig. 6,is formed with a recessed inner end, in accordance with the disclosureof the Smith patent above mentioned, so that any conductive film likelyto be deposited on the surface of this insulator by metal sputtered orevaporated during fabrication or operation of the tube will not becontinuous and provide an unwanted conductive path between the grid wire28 and the Shield S.

An arrangement of battle plates is included in the lower part of theanode shield S below the grid G for the purpose of preventing mercurybeing splashed up against the heated walls of the shield duringoperation, and to impede the movement of the electrons and ions in theplasma formed by the keep-a1ive anode into the region directly under thecontrol grid to affect its controlling influence. In the particularbaii'le structure illustrated, a lower baliie plate 34, with a centercircular opening indicated at 35, has a peripheral flange welded to theinside Wall of the shield S (see Fig. l). A cruciform arrangement ofupright strips 36 welded to this lower baffle plate 34 supports anintermediate circular baffle plate 37, which has a diameter slightlygreater than the diameter of the opening 35 in the lower baffle plate.Above this intermediate baffle plate 37 is an upper baffle plate 3%,similar to the lower baffle plate 34, which has a center circularopening 39, and is secured by a welded peripheral flange to the shieldS.

in the particular tube structure illustrated, the keepalive anode KAcomprises an. arcuate strip 42, supported by insulating supportingelements from the lower edge of the anode shield S, as shown in Figs. 1and 4. Each of the insulated elements, three as shown, for supportingthe keep-alive anode strip 42 from the shield S, comprises a rod 43 (seeFig. 1) having its upper end welded to the shield S, and a tubularinsulator 44 of steatite or like material around this rod, which issecured to the anode strip 42 by a strap 45 seated in a peripheralrecess in said insulator and welded to the keep-alive anode strip. Asuitable projection on the rod 43 to consittute a stop engaging thelower end of the insulator 44 and hold up the keep-alive anode strip 42is provided by deformation or bending of this rod 43, or by anadditional welded piece, as indicated at 46.

The external connection to the keep-alive anode KA comprises arelatively stiff wire or rod 50 which is sealed at its upper end in theusual seal (not shown) in the upper part of the glass body E. This wire50 is bent to form a coil or winding of several turns around but spacedfrom the tubular extension of the shield S around the anode lead, forreasons later explained; and this wire 50 then extends down to a pointwhere it is attached by a welded tab 51 to the keep-alive anode strip42. The portion of this wire 50 adjacent the shield is preferablysurrounded by a tube 52 of steatite or like heat resistant insulatingmaterial.

The automatic starting switch comprises a flipper armature 55 pivotallysupported on a horizontal axis to be swung from a vertical position incontact with the mercury, as shown in Fig. l, to produce a starting are,by energization of an electromagnet EM supported in a convenient mannernot specifically shown outside the tube close to the tube envelopeopposite this flipper armature, as diagrammatically represented inFig. 1. The pivotal support for this flipper armature 55 is preferablyformed to involve loosely fitted parts of a metal, such as molybdenum,having a high melting point, so that the high temperature these partsmay assume in operation will not cause any fusing or welding effect atthe contacting surfaces to interfere with free movement of the armature.In the structure illustrated, the flipper armature 55 is pivotallysupported by a pair of eyelets cooperating with a rod 56 secured in aflanged lip of a supporting plate 57. These eyelets are convenientlyformed at the ends of a wire loop 58 which is seated in grooves in thesides and bottom of the flipper armature 55, and which provides ineffect a cradle to support the block of magnetic material constitutingthe armature. The supporting plate 57 is welded to the bent lower end ofa lead-in rod 60 of substantial stiffness, which is surrounded by asleeve 61 of steatite or the like to insulate it from the shield S. Thelower end of this lead-in rod 60 and its insulating sleeve 61 setsbetween a pair of cars 62 welded to the shield S. A pair of bumpersteatite insulators 64 are secured by welded bands 65 to the supportingplate 57 in a position such that when these insulators engage glass bodyE of the tube envelope, the armature 55 is in the appropriate positionto be oerated by the electromagnet EM to the best advantage withoutstriking the glass wall. In this connection, the dimensions of the glassbody B will vary somewhat for different tubes, and a flipper armature 55pivoted in a fixed relation to a shield S would not necessarily be inthe proper position relative to the glass wall. In the structure shown,the lead-in supporting rod 60 is bent during assembly to have a tendencyto move outward radially at its lower end; and when the parts areassembled in a glass body E of a given size, the bumper insulators 64engage the wall of this glass body (see Fig. 4) and establish thedesired position for the flipper armature. For convenience in assemblythe lead-in supporting rod 60 for the flipper armature is preferablymade in two parts welded together, as shown in Fig. 1.

Considering the operation of the automatic starting switch, the flipperarmature 55 tends to drop by its own weight into the vetrical positionwhere its lower edge dips in the mercury pool. Referring to the circuitdiagrams of Figs. 7 and 8, when the tube is put into use by closing aswitch indicated at 67, uni-directional current from a suitable sourceof direct current or rectified alternating current of the appropriatevoltage for the keep-alive arc flows through a load resistor 68, astarting resistor 70, to the flipper armature 55 and pool of mercury,and through the windings of the electromagnet EM. The resultantenergization of the electromagnet EM attracts the flipper armature 55fromthe position shown in Fig. 7 to an approximately horizontal positionshown in Fig. 8. This movement of the flipper armature 55 draws astarting arc; and due to the starting resistance in series with thisare, there is a higher voltage across the keep-alive anode KA and thecathode pool, so that the starting arc is transferred to the keep-aliveanode KA, with the current of the keep-alive discharge also flowingthrough the winding of the electromagnet EM. In this connection, thecoils of the electromagnet EM are preferably wound and connected tocreate a magnetic field in a direction to repel the starting are betweenthe flipper armature 55 and the cathode pool toward the keep-alive anodeKA, in accordance with the principles of the well known are blow-outdevices. So long as an effective keep-alive discharge are exists,between the cathode pool and either the keep-alive anode KA or theflipper armature 55, the electromagnet EM is maintained energized tohold the flipper armature in its raised position. If, however, asuitable keep-alive discharge ceases to exist, the electromagnet EM isdeenergized, and the flipper armature 55 drops by its own weight intocontact with the mercury pool to establish a conductive circuit forenergization of the electromagnet, whereupon the starting arc is againinitiated. In this arrangement, the electromagnet EM operating to drawthe initial starting are also serves to detect the existence of akeep-alive discharge, thereby avoiding the need of an additional relayfor this purpose.

Considering now the functional significance of the features of tubestructure illustrated and described, and the characteristic features ofthis invention, one important feature of the invention relates to theanode shielding to permit the grid G to exercise its desired controllingfunction at high anode voltages. In a grid control tube, as distinctivefrom an igniter type of tube, the cathode is normally active, so tospeak, and the tube will conduct whenever its anode is positive unlessthe grid is capable of preventing initiation of cumulative ionizationand an arc discharge. In general, a control grid may influence theionizing effect of lines of force of the electric field of a positiveanode that pass through the grid, but has little effect upon ionizationby the anode electric field along other paths. For example, lines offorce from the side or back surfaces of the anode, or from itssupporting lead, may initiate an arc discharge in spite of thecontrolling effect of the grid. Such uncontrolled paths of ionizationmay be relatively long; and for a given set of conditions in the way ofthe concentration and state of the gas molecules, electrode surfacecondition and the like, positive anode voltages above a certain levelcan render the tube conductive in spite of the grid control, imposingundesirable limitations upon the peak forward voltage rating for thetube. In the case of a grid control mercury pool tube with a keepaliveanode, the vapor subject to the ionizing influence of anode potentialsbeyond the control of the grid is likely to be partially ionized fromthe plasma of the keep-alive anode, and materially limit the peakforward voltage for the tube.

In accordance with this invention, an acceptable peak forward voltageand effective grid control are obtained by completely shielding allsurfaces of the anode and associated parts at anode potential, exceptfor an area directly opposite the grid, from the cathode and the plasmaof the keep-alive anode, by an anode shield S constructed as shown anddescribed, which is maintained at cathode or other appropriatepotential. Since breakdown along any ionizing path, no matter how longor restricted its cross section may be, is likely to cause an unwantedconduction through the tube, complete shielding of surfaces at anodepotential is important. Various types of anode shields have beenpreviously proposed, but all of the structures with which I am familiarfail to provide the necessary complete shielding for satisfactoryresults.

In the tube structure of this invention, the anode shield S is shaped toshield the edge and back of the anode A, the entire length of the tube12 around the anode supporting lead 5, and is also closely spaced to theouter edge of the grid G, so that all points on surfaces at anodepotential, except directly opposite the grid G, are isolated by theshield S from the cathode and ionized vapor in the tube envelope.

The addition of anode shielding to enable the grid control to beeffective for higher anode voltages introduces the problem of avoidingbreakdown and an arc discharge as between the anode and such shield. Theanode shielding structure of this invention is closely spaced to thesurfaces at anode potential, in accordance with the well knownphenomenon in gaseous discharges, sometimes termed the mean free pathprinciple, that the space or distance between electrode surfaces may bemade small enough with respect to the pressure, type of gas and otherfactors to a degree where effective ionization and breakdown betweensuch electrode surfaces requires extremely high voltages. Theappropriate spacing effective to avoid breakdown is dependent uponvarious complex factors, and is diflicult to define. From one point ofview, it may be said that breakdown will not occur if the spacingcorresponds with the electron mean free path for the existing pressure;and the appropriate spacing is sometimes termed a mean free pathspacing. However, ionization sufficient to cause a sustained aredischarge of substantial current involves other factors than theprobability of ionizing collisions between electrons and gas molecules;and in general, effective results may be obtained with a spacing greaterthan the theoretical electron mean free path for the existing gaspressure. For the purposes of this case, it is proposed to refer to thespacing for the anode shield S characteristic of this invention as ashort path spacing, or a space or distance in the order of the electronmean free path for the operating vapor pressure. In this connection, byway of explanation and without limiting the invention, I have found thata spacing for the cathode shield S in the order of 2 mm. is appropriatefor typical operating conditions of the type of tube illustrated.

At this point in the discussion it is convenient to refer to anotherstructural problem presented by the use of complete anode shielding,which involves maintaining the desired short path space relationship atand around the upper end of the anode shield S adjacent the seal for theanode lead, and still obtain the necessary electrical isolation of theshield for high anode voltages. In the structure illustrated, the glassbody E is formed with a re-entrant tubular extension or neck 72 adjacentthe cup 10 forming the metal-to-glass seal for the anode supporting rod5 and its surrounding tube 12. The upper tubular portion of the shieldS, which is closely spaced to the tube 12 at anode potential throughoutits length, extends up into this glass neck 72, where it is surroundedby a packing of quartz wool, as indicated at 73, or like heat resistantinsulating material of a fibrous or finely dividing nature. The glassneck '72 is preferably tapered slightly to hold the quartz wool packing73 in place.

Considering now the significance of this structural feature of the anodeshielding of this invention, the shield S as a whole may be readilysupported in an insulated space relationship to the anode A andassociated parts in a suitable manner, such as by rods 15 sealed in theglass body E as shown, with suflicient rigidity to maintain the desiredshort path spacing and electrical isolation for most of the surfaces atanode potential. There is, however, a region adjacent the seal of theanode lead in the tube envelope where a special structure is needed tomaintain the short path spacing for complete shielding, and at the sametime provide adequate electrical insulation for the shield. In otherwords, there should be no gap between the upper edge of the shield S andthe envelope wall adjacent the anode lead, otherwise there will be along path for unwanted ionization from the anode lead through such gapto the outside surface of the shield; yet the separation of the parts isso small for the differences of potential involved that adequateelectrical isolation cannot be obtained by merely extending the shieldinto the wall of the envelope adjacent the seal for the anode, sinceglass or other conventional insulating materials for the envelope wallwould not have in such short dimensions the requisite dielectricstrength for the differences of potential involved. In the structure ofthis invention, the quartz wool packing 73, around the upper edge of theshield S affords the desired electrical insulation, on account of thehigh dielectric strength of quartz even at high temperatures, and theinterstices or spaces between fibers or pieces of the quartz are sosmall and so disconnected that there are no long ionizing paths for agaseous discharge between the shield S and the tube 12 at anodepotential.

As previously explained, the keep-alive anode current is conductedthrough the wire 50 coiled around the upper part of the shield Sadjacent the anode lead. Considering the purpose of this auxiliaryheating coil, the anode A and in turn the upper tubular portion of theshield S are kept hot enough while the tube is conducting to avoid anycondensation of the mercury vapor, which migl t otherwise establish ashort-circuiting electrical connection between the shield and anode,more particularly in the region of the quartz wool packing 73; but ifthe tube is idle for some time with the discharge limited to thekeep-alive current, it is found that the anode and its associated tube12 and the shield S may assume a temperature low enough for condensationof mercury vapor to a degree to set up conditions for a short-circuitbetween tube 12 at anode potential and shield S. The auxiliary heatingcoil obviates these conditions by maintaining the parts at a high enoughtemperature by the keep-alive anode current while the tube is inservice, but not conducting to the main anode.

Referring now to the arrangement of bafiie plates 35, 37, 38 in thelower part of the anode shield S, the parts of this bathe system arearranged and proportioned to carry out physical and electricalfunctions. While the tube is conducting, movement of the cathode spotand other changes tend to produce spurts of mercury, which except forthe baffle system might splash on the hot wall of the shield S in theregion under the grid G, and cause an abrupt change in pressure byevaporation of such mercury, tending to affect the controlling influenceof the grid. The bafile plates act as barriers to keep spurts of mercuryfrom reaching the region under the grid G, any evaporation or splashedmercury occurring within the battle system, where the increase pressuremay quickly stabilize with the region of low pressure in the cathoderegion.

Considering the electrical function of the baffle plates 35, 37, 38,while conduction through the tube is intended to be prevented by thecontrol grid G, an arc discharge is maintained between the keep-aliveanode KA and the cathode, and except for the baffle system, the plasmafor the keep-alive discharge would extend into the region under the gridG, and there would be a drift of positive ions to the grid tending toreduce its negative potential, and cause unwanted conduction. The partsof the battle system are proportioned to establish a boundary conditionfor the plasma of the keep-alive discharge by affording sufiicientsurface for deionization to keep the ion concentration in the regionbelow the grid G at a level appropriate for aceptable grid control. Inthis connection, a bafiie system of this character raises the startingvoltage for the tube, i. e. the anode voltage which will initiateconduction at a zero grid potential, so that a baffie systemproportioned to keep the plasma of the keep-alive discharge out of theregion underneath the grid is likely to require a positive gridpotential to fire the tube at the desired level of anode voltage. It iscontemplated that the baffle system will be proportioned to provide acompromise between the desired grid control for peak forward voltageratings and starting voltage, and that the tube will be controlled byabruptly changing the grid from a negative potential to a relativelyhigh positive potential.

The tube structure of this invention provides a relatively high andacceptable inverse voltage rating for the tube, since the completeshielding of the back of the anode and its supporting lead eliminatesthe ionizing paths existing in the ordinary tube structures and capableof causing initiation of an arc discharge through the tube in the wrongdirection, when the alternating supply voltage commonly used withrectifier tubes makes the anode at a high negative potential relative tothe cathode. The shielding effect of the anode shield S, and thearrangement of baffie plates as between the cathode and the lowersurface of the anode, together with the interposed grid G at a negativepotential with respect to the cathode, tends to avoid effectiveionization and initiation of a discharge in the wrong direction betweenthe cathode and the lower surface of the anode. In this connection, thegrid G is closely spaced to this lower surface of the anode to reducethe probability of ionization in the space and the formation of an ionsheath on the grid for high differences of potential between the anodeand the grid, even though these electrode surfaces may be somewhatemissive and there may be appreciable electron current. It iscontemplated that this close spacing will conform with the short ormean-free path spacing for the existing pressure or concentration of gasmolecules to a degree avoiding effective ionization. In addition toraising the inverse voltage rating for the tube, this close grid toanode spacing minimizes the probability of loss of grid control due togrid emission.

In general, the cooperative effect of these various structural featuresserves to provide a high level of forward and inverse voltage ratingsfor the tube, and limit to an acceptable degree the probability ofbackfires or arc-backs characteristic of a mercury pool tube, when thetube is operated within the limits of its voltage and current ratings.

The specific embodiment of the invention just described exemplifies thestructural features and functions characteristic of the invention; butit should be understood that this particular construction andarrangement of parts is merely typical or representative, and thatvarious adaptations, modifications and additions may be made withoutdeparting from the invention.

What I claim is:

l. A gaseous discharge tube comprising, an evacuated envelope containingan ionizable medium and 2. normally active cathode, an anode having anelectron receiving area opposite said cathode, an anode supporting leadsealed in said envelope, an imperforate metal shield completelysurrounding said anode and its lead except for said electron receivingarea of the anode, and means rigidly supporting said shield within theenvelope in a short path space relationship to all points on theopposing surfaces of said anode and its lead, said short path spacingbeing in the order of two millimeters to inhibit an arc discharge tosaid shield at high anode voltages, said supporting means including afibrous body of refractory insulating material located at the gapbetween said shield and the anode lead adjacent its seal in the tubeenvelope, said insulating fibrous body having its 1O conductivecontinuity interrupted by interstices smaller than said short pathspacing, whereby an arc discharge to said shield from the anode lead isinhibited in the region adjacent its seal as well as throughout itslength.

2. A controllable gaseous discharge tube comprising, an evacuatedenvelope containing an ionizable medium, an anode having a supportinglead sealed in said envelope, a normally active cathode, a grid betweensaid cathode and an electron receiving surface of said anode, animperforate metal shield completely surrounding and having a short pathspacing to all points on the surfaces of said anode and its supportinglead except said electron receiving surface, said short spacing being inthe order of two millimeters to avoid a breakdown between said shieldand the anode and its lead at relatively high anode voltages and thetemperature and vapor pressure conditions existing during operation ofthe tube, and an insulated joint between portions of said shield andsaid anode lead in said short path space relationship, said jointincluding a fibrous body having a breakdown voltage much higher thansolid insulating material of like dimensions.

3. A grid control gaseous discharge tube having a mercury pool cathodeand comprising, an evacuated envelope including a metal bottomsupporting the mercury pool cathode and a glass body sealed to saidmetal bottom, an anode having a supporting lead sealed in said glassbody, a control grid between said anode and cathode, an imperforateshielding element rigidly supported in said envelope with inner surfaceshaving a short path spacing to the periphery of said grid and to allpoints on the surfaces of said anode and its lead except for a dischargereceiving area opposite said grid, said short path spacing being lessthan three millimeters to avoid a breakdown at relatively high anodevoltages between said shielding element and said anode and its lead atthe temperature and vapor pressure existing with air cooling of the tubeenvelope, and an insulating connection between a portion of saidshielding element and said anode lead, said connection including afibrous body having a breakdown voltage much higher than a solidinsulating material of like dimensions.

4. A grid control mercury pool rectifier tube comprising, an air-cooledenvelope having a metal bottom and a glass body sealed together, ananode having a supporting lead sealed in said glass body of theenvelope, a mercury pool cathode supported by the metal bottom of saidenvelope, a control grid between the cathode and anode, a keep-aliveanode and starting device in said envelope and a metal shield enclosingsaid anode and said grid, said shield having a short path spacing to allpoints on the surfaces of said anode and its supporting lead except foran area opposite the grid, said shield also including baflie platesbelow said grid for restricting movement of ions associated with thekeep-alive discharge into the region of said grid. I

5. A gaseous discharge tube of the mercury pool type comprising, anevacuated envelope having a metal bottom supporting a mercury poolcathode, an anode having a supporting lead sealed in said envelope, ananode shield having a short path spacing to the surface of said anodesupporting lead, a control grid opposite said anode, a keep-alive anodein said envelope and an auxiliary heating coil around said shieldadjacent said anode supporting lead and connected with said keep-aliveanode, whereby the closely spaced surfaces of the anode and its shieldare maintained at a temperature avoiding condensation of mercury bykeep-alive anode current through said auxiliary heating coil under lowload or stand-by conditions of tube operation.

6. A controllable gaseous discharge tube comprising an evacuatedenvelope containing an ionizable medium,

' an anode having a supporting lead sealed in said envelope, a controlgrid opposite an area of the anode surface, a metallic shield having ashort path spacing to said grid and to all surfaces of said anode andits supporting lead except the area opposite the grid, and a jointstructure 'at the end portion of said shield adjacent the seal of saidanode lead in the envelope maintaining short path spacing and electricalinsulation, said joint structure including a packing of fibrous heatresistant insulating material between the closely spaced surfaces ofsaid shield and anode lead in a region adja' cent the'seal of said anodelead in the envelope.

7. A gaseous discharge tube according to claim 6, in which the endportion of the anode shield extends into a re-eiitr'ant neck adjacentthe seal of the anode lead in the envelope, and is surrounded by apacking of quartz wool.

8. A controllable gaseous discharge tube of the mercury pool typecomprising, an evacuated envelope having a metal bottom supporting amercury pool cathode and a glass body sealed to said bottom, an anodehaving a supporting lead sealed in said glass body of the envelope,

a control grid adjacent an area of the anode opposing the mercury poolcathode, a metal shield closely spaced to the periphery of said grid andall surfaces of said anode and its supporting lead except the area ofsaid control' grid, a keep-alive anode supported by but insulated fromsaid shield, and an automatic starting de- \"ice comprising a flipperarmature dipping into the cathode pool in a normal position, anelectromagnet mounted adjacent said starting device for swinging saidarmature from its normal position to provide a starting arc to betransferred to the keep-alive anode, and means for energizing said'electromagnet by keep-alive anode current, whereby the automaticoperation of said starting device is dependent upon a maintainedkeep-alive discharge.

9. A gaseous discharge tube of the mercury pool type comprising anair-cooled evacuated envelope enclosing a control grid interposedbetween a mercury pool cathode and a shielded anode, said envelopeincluding a metal base supporting the cathode pool and a glass bodysealed to said base, said base having a peripheral trough to receive theglass body, the inner side wall'of said trough being spaced from theglass, whereby said side wall of the trough may yield upon distortion ofthe metal base to avoid stress upon the seal between the glass and thebottom of the trough.

10. A controllable gaseous discharge tube of the mercur'y pool 'typecomprising, an evacuated envelope enclosing an anode and a normallyactive cathode, a supporting lead for said anode sealed in saidenvelope, a control grid between said anode and said cathode and closelyspaced to an electron receiving surface of said anode, said control gridcomprising a supporting ring and a plurality of spaced grid bars, eachof said grid bars being anchored to the supporting ring at one end onlyand having a slidable fit at its other end between guiding elements'onsaid ring, and an imperforate metal shield having a short path spacingto the periphery of said grid and to all surfaces of said anode and itssupporting lead except for the electron receiving surfaceoppositesaid'grid.

11. A controllable gaseous discharge tube comprising, an evacuatedenvelope containing an ionizable medium, an imperforate metal shieldsupported in said envelope, an anode and a control grid within saidshield and havihg a short path spacing thereto in the order of theelectron mean free path of said ionizable medium, and means includingtubular insulators or refractory material disposed in different planessupporting and conductively insulating said grid from said shield at aplurality of points around the periphery of the grid, said grid beingmaintained by said supporting means against displacement in a closelyspaced relationship 'to an electron receiving surface of said anodeopposite said grid.

12. A controllable gaseous discharge tube of the mercury pool typecomprising, an air-cooled envelope having a metal bottom supporting amercury pool-cathode,

an anode, a control grid between said cathode and said anode, akeep-alive anode, and an imperforate metal shield rigidly supported insaid envelope around said anode and said grid, said shield having ashort path spacing to surfaces of said anode and grid in the order ofthe electron mean free path for the operating pressure of mercury vapor,said shield having bafiie plates establishing a boundary for the plasmaof the keep-alive discharge as between said mercury pool cathode andsaid grid.

13. A controllable gaseous discharge tube of the mercury pool typedescribed comprising, an air-cooled envelope having a metal bottomsupporting a mercury pool cathode, a radiator attached to said metalbottom outside the envelope for dissipating heat to provide a relativelylow temperature in the cathode region and maintain a low pressure ofmercury vapor during operation of the tube, an anode and a control gridin said envelope, a keep-alive anode, and an imperforate metal shieldand attached baffle plates isolating said anode and control grid fromthe plasma of the keep-alive discharge except for crooked paths throughsaid baffle plates, said shield establishing a boundary of said plasmapreventing the formation of an ion sheath on the surface of the gridwhen at a negative potential.

14. A gaseous discharge tube of the character described. comprising, anevacuated envelope having an aircooled metal bottom supporting a mercurypool cathode, a main anode, a control grid, a metal shield enclosingsaid anode and grid and preventing an arc discharge to the anode notsubject to the control of said grid, a keepalive anode and startingdevice normally maintaining an arc discharge to the mercury pool cathodeto be transferred to the main anode under the control of said grid, saidstarting device comprising a flipper armature biased to a normalposition in contact with the mercury of the cathode, an electromagnetoutside the envelope for attracting said armature to a position out ofcontact with the mercury to provide a starting arc, and an energizingcircuit for said electromagnet including circuit paths in multiplebetween said keep-alive anode and said flipper armature respectively andthe cathode, said circuit path through said armature including aresistance to cause the starting arc to be transferred to the keep-aliveanode.

15. A controllable mercury arc rectifier tube comprising, an exacuatedenvelope including an air-cooled metal container for a shallow cathodepool of mercury and a glass body sealed to said container, an anodesupported by a lead-in rod sealed in said glass body, a control grid,shielding means including imperforate surfaces having a short pathspacing to the periphery of said grid and to all points on the surfacesof said anode and its lead-in rod except for a discharge receiving area,a keepalive anode and a starting device, and auxiliary heating meansenergized by the discharge current of the keepalive anode for heatingsaid shielding means above the temperature for condensation of mercuryunder low load or stand-by conditions of tube operation.

16. A controllable mercury arc rectifier tube comprising, an evacuatedenvelope including a container of thin metal for a shallow cathode pool,a heat radiator directly attached to the outside surface of saidcontainer, an anode supported by a lead-in rod sealed in said envelope,a control grid closely spaced to a discharge receiving area of saidanode, a keep-alive anode and starting device, and means includingimperforate shielding surfaces and a baffle system for completelyisolating said anode and said grid from the plasma of the auxiliarydischarge to the keepalive anode except for crooked passages through thebaffle system to a region adjacent said grid inside said shieldingsurface, said shielding surface being closely spaced to the periphery ofsaid grid and to all points on the surface of said anode and its lead-inrod except for the discharge receiving area of the anode.

17. An air-cooled controllable mercury arc rectifier tube Ase;

comprising, an evacuated envelope including a body of vitreous materialand a metal container sealed thereto for supporting a shallow pool ofmercury, an anode and a control grid supported in said envelope withseparate lead-in connections and having closely spaced opposingsurfaces, imperforate shielding means having inner surfaces closelyspaced to the periphery of said grid and to all points on the surfacesof said anode and its lead-in connection except for a dischargereceiving area opposite said grid, a keep-alive anode and startingdevice for establishing an auxiliary arc discharge to be transferred tothe main anode subject to the control of said grid, and bafiling meansconnected with said shielding means and comprising surfaces separated bycrooked passages to restrict movement of the plasma of the auxiliarydischarge of the keep-alive anode into the region of the grid.

18. A controllable mercury arc rectifier tube comprising, an envelopeincluding an air-cooled metal base supporting the cathode pool and aglass body sealed to said base, an anode supported by a lead-in rodsealed in said glass body and having an essentially flat dischargereceiving surface, a control grid closely spaced to said dischargereceiving surface of the anode, said grid comprising spaced grid bars ofrefractory metal rigidly supported at one end only and capable ofexpanding in length without bending, and an imperforate metal shieldsupported in said envelope and having a short path spacing to theperiphery of said grid and to all points on the surfaces of said anodeand its lead-in rod except for said discharge receiving surface of theanode.

19. An air-cooled mercury arc controllable rectifier tube comprising, anenvelope of glass sealed to a relatively thin metal base supporting thecathode pool, an external radiator of spaced strips having their edgesdirectly attached to the outer surface of said base, a carbon anodehaving a flat discharge receiving surface and supported by a leadin rodsealed in the glass body of the tube envelope, a control grid includingspaced grid bars of refractory metal having their edges closely spacedto the discharge receiving surface of said anode, said grid bars havinga width greater than their spacing, and an imperforate metal shieldsupported in said envelope with substantial rigidity and having itsinner surface closely spaced to the periphery of said grid and to allpoints on the surfaces of said anode and its supporting lead except thedischarge receiving surface of the anode.

20. A controllable mercury arc rectifier tube comprising, an air-cooledenvelope including a glass body sealed to a metal base supporting themercury pool cathode, an anode supported by a lead-in rod sealed in saidglass body, a control grid closely spaced to a discharge receivingsurface of said anode, an imperforate metal shield supported in saidenvelope and having a short path spacing to the periphery of said gridand to all points on the surfaces of said anode and its lead-in rodexcept for said discharge receiving surface, a keep-alive anode, astarting device including a flipper armature movable into and out ofcontact with the mercury pool cathode, and means including heatresistant insulating elements supporting from said shield said grid,keep-alive anode and starting device.

References Cited in the file of this patent UNITED STATES PATENTS2,067,966 Klemperer Jan. 19, 1937 2,101,610 Dallenbach Dec. 7, 19372,221,615 Slepian Nov. 12, 1940 2,282,229 Longwell May 5, 1942 2,320,685Bertele June 1, 1943 2,392,367 Depp Jan. 8, 1943 2,477,506 Winogard July26, 1949 2,573,618 Smart Oct. 30, 1951 2,594,851 Bertele Apr. 29, 1952FOREIGN PATENTS 665,469 Great Britain J an. 23, 1952

